Apparatus for control of deposit build-up on an inner surface of a plasma processing chamber

ABSTRACT

A method and apparatus for controlling deposit build-up on an interior surface of a dielectric member of a plasma processing chamber. The deposit build-up is controlled by selective ion bombardment of the inner surface by shifting location of a peak voltage amplitude of a voltage standing wave on an antenna such as a flat spiral coil of the plasma processing chamber. A region of high ion bombardment on the interior surface of the dielectric member is displaced by controlling the value of a termination capacitance over a range of values causing regions of low and high ion bombardment to move over the dielectric member in order to effect cleaning thereof.

This application is a divisional, of application Ser. No. 08/828,507,filed Mar. 31, 1997.

FIELD OF THE INVENTION

The invention relates to improvements in a plasma reactor and to amethod of processing a substrate in the plasma reactor such as by plasmaetching an oxide layer on a semiconductor wafer.

BACKGROUND OF THE INVENTION

Vacuum processing chambers are generally used for chemical vapordepositing (CVD) and etching of materials on substrates by supplyingprocess gas to the vacuum chamber and application of an RF field to thegas. Examples of parallel plate, transformer coupled plasma (TCP™, alsocalled ICP), and electron-cyclotron resonance (ECR) reactors aredisclosed in commonly owned U.S. Pat. Nos. 4,340,462; 4,948,458; and5,200,232. The substrates are held in place within the vacuum chamberduring processing by substrate holders. Conventional substrate holdersinclude mechanical clamps and electrostatic clamps (ESC). Examples ofmechanical clamps and ESC substrate holders are provided in commonlyowned U.S. Pat. No. 5,262,029 and commonly owned U.S. application Ser.No. 08/401,524 filed on Mar. 10, 1995. Substrate holders in the form ofan electrode can supply radiofrequency (RF) power into the chamber, asdisclosed in U.S. Pat. No. 4,579,618.

Plasma reactors wherein an antenna coupled to a radiofrequency (RF)source energizes gas into a plasma state within a process chamber aredisclosed in U.S. Pat. Nos. 4,948,458; 5,198,718; 5,241,245; 5,304,279;5,401,350; 5,531,834; 5,464,476; 5,525,159; 5,529,657; and 5,580,385. Insuch systems, the antenna is separated from the interior of the processchamber by a dielectric member such as a dielectric window, gasdistribution plate, encapsulating layer of epoxy, or the like, and theRF energy is supplied into the chamber through the dielectric member.Such processing systems can be used for a variety of semiconductorprocessing applications such as etching, deposition, resist stripping,etc.

During an oxide etch of a semiconductor wafer in a plasma reactor,polymer can build up on the exposed surface of the dielectric member. Asthe polymer build-up deepens the uniformity of processing of thesubstrate can be affected and/or polymer can then flake off of thedielectric member. If the dielectric member is located directly abovethe substrate and chuck, polymer particles can fall directly on thesubstrate or the chuck below. This can ruin the substrate decreasingyield or cause chucking problems. In addition, the process must bestopped and the chamber cleaned. The delay due to the "down-time"required for cleaning also represents a substantial loss in productionyield. Therefore, control of the deposition of polymer on the dielectricmember is critical for achieving a high yield and maintainingthrough-put of the substrates in the plasma reactor.

SUMMARY OF THE INVENTION

An object of the present invention is to increase through-put ofprocessed substrates when substrates such as semiconductor wafers areprocessed continuously in a plasma reactor with a surface of adielectric member such as a dielectric window or gas distribution plate(GDP) facing the substrate.

It is a further object of the invention to control deposit build-up onan interior surface in a plasma reactor and thereby increase the yieldof the produced substrates and reduce down-time need for cleaning of thereactor.

The foregoing and other objects are accomplished by an improved methodand apparatus for controlling the voltage standing wave exhibited by anantenna separated from the interior of a plasma reactor by a dielectricmember. According to one aspect of the present invention, it has beenobserved that in the region or regions of high amplitude of the voltagestanding wave of the antenna, deposits are cleaned from the dielectricmember by ion bombardment. On the other hand, at regions along theantenna where the amplitude of the standing wave voltage is low,deposits will build up a thick, sometimes poorly adherent layer.Therefore according to one embodiment of the present invention, theregion of ion bombardment is moved around by displacing the standingvoltage wave of the antenna such that dielectric member is "cleaned" orany deposit build-up that does occur is inhibited and more uniform.

According to one exemplary embodiment of the invention, the value of thetermination impedance of the antenna is varied to control the locationof the high and low voltage amplitudes of the standing wave on theantenna. According to this embodiment of the present invention, thevalue of a termination capacitor is varied such that regions of low andhigh ion bombardment move over the dielectric member.

According to an exemplary embodiment, when the termination capacitor isat a high value, a clean central region is formed and deposit build-upoccurs in an outer region surrounding the central region. At lowercapacitive values, the central region exhibits deposit build-up and theouter region is cleaned by ion bombardment. By controlling ionbombardment of these regions, the uniformity and adherence of thedeposit build-up on the dielectric member may be controlled.

According to an exemplary embodiment, the termination capacitor may bevaried two or more times, continuously varied or set to a desired valueduring processing of an individual substrate such as during an oxideetch of a semiconductor (e.g., silicon) wafer in the plasma reactorand/or during a clean cycle following the etching step. For instance,the termination capacitor can be operated at one value during theprocessing (e.g., during a wafer etch process) and the capacitor can beoperated at a different value during a cleaning step (e.g., during anoxygen clean cycle).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings in which like elements bear like referencenumerals, and wherein:

FIG. 1 is a cross sectional view of a vacuum processing chamber having aliner, focus ring and gas distribution plate according to an exemplaryembodiment the invention;

FIG. 2 is a top view of an 89 hole gas distribution plate according toan exemplary embodiment the invention;

FIG. 3 illustrates an antenna in the form of a flat spiral coilaccording to an exemplary embodiment of the invention;

FIGS. 4a-4b are representations of deposit build-up patterns (shown withcross-hatching) with variation in termination capacitance;

FIG. 5 illustrates an exemplary circuit arrangement for an antenna inthe form of the spiral coil shown in FIG. 3;

FIG. 6 illustrates the effect of variations of the C₄ value of thecircuit shown in FIG. 5 on values of C₁, C₂ and C₃ which minimizereflected power;

FIG. 7 illustrates the effect of variations of the C₄ value on theantenna voltage amplitude and the bottom electrode RF voltage; and

FIG. 8 illustrates the effect of variations of the C₄ value on theantenna current amplitude and the bottom electrode RF current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides improvements in reducing particle contaminationof substrates such as semiconductor (e.g., silicon, gallium arsenide,etc.) wafers, flat panel display substrates, and the like. In addition,the invention provides uniform and reproducible processing ofsequentially processed substrates while allowing extremely longcontinuous processing runs (e.g., over 100, preferably over 1000 andmore preferably over 2000 wafer runs) between shut downs for reactorcleaning.

In plasma etching of substrates, features can be etched into layers ofvarious materials on substrates such as silicon wafers. In such etchingprocesses, a gas distribution plate can be used to control the spacialdistribution of gas flow in the volume of the reactor above the plane ofthe substrate. In the TCP™ 9100™ plasma etching reactor available fromLAM Research Incorporated, the gas distribution plate is a circularplate situated directly below the TCP™ window which is also the vacuumsealing surface at the top of the reactor in a plane above and parallelto a semiconductor wafer. The gas distribution plate is sealed using anO-ring to a gas distribution ring located at the periphery of the gasdistribution plate. The gas distribution ring feeds gas from a sourceinto the volume defined by the gas distribution plate, an inside surfaceof a window underlying an antenna in the form of a flat spiral coilsupplying RF energy into the reactor, and the gas distribution ring. Thegas distribution plate contains an array of holes of a specifieddiameter which extend through the plate. The spacial distribution of theholes through the gas distribution plate can be varied to optimize etchuniformity of the layers to be etched, e.g., a photoresist layer, asilicon dioxide layer and an underlayer material on the wafer. Thecross-sectional shape of the gas distribution plate can be varied tomanipulate the distribution of RF power into the plasma in the reactor.The gas distribution plate material must be a dielectric to enablecoupling of this RF power through the gas distribution plate into thereactor. Further, it is desirable for the material of the gasdistribution plate to be highly resistant to chemical sputter-etching inenvironments such as oxygen or a hydrofluorocarbon gas plasma in orderto avoid breakdown and the resultant particle generation associatedtherewith.

A vacuum processing chamber according to one embodiment of the presentinvention is illustrated in FIG. 1. The vacuum processing chamber 10includes a substrate holder 12 in the form of a bottom electrodeproviding an electrostatic clamping force to a substrate 13 as well asan RF bias to a substrate supported thereon and a focus ring 14 forconfining plasma in an area above the substrate while it is Hebackcooled. A source of energy for maintaining a high density (e.g. 10¹¹-10¹² ions/cm³) plasma in the chamber such as an antenna 18 in the formof a flat spiral coil powered by a suitable RF source and suitable RFimpedance matching circuitry inductively couples RF energy into thechamber 10 so as to provide a high density plasma. The chamber includessuitable vacuum pumping apparatus for maintaining the interior of thechamber at a desired pressure (e.g. below 50 mTorr, typically 1-20mTorr). A substantially planar dielectric window 20 of uniform thicknessis provided between the antenna 18 and the interior of the processingchamber 10 and forms the vacuum wall at the top of the processingchamber 10. A gas distribution plate, commonly called a showerhead 22,is provided beneath the window 20 and includes a plurality of openingssuch as circular holes (not shown) for delivering process gas suppliedby the gas supply 23 to the processing chamber 10. A conical liner 30extends from the gas distribution plate and surrounds the substrateholder 12. The antenna 18 can be provided with a channel 24 throughwhich a temperature control fluid is passed via inlet and outletconduits 25,26. However, the antenna 18 and/or window 20 could be cooledby other techniques such as by blowing air over the antenna and window,passing a cooling medium through or in heat transfer contact with thewindow and/or gas distribution plate, etc.

In operation, a wafer is positioned on the substrate holder 12 and istypically held in place by an electrostatic clamp, a mechanical clamp,or other clamping mechanism when He backcooling is employed. Process gasis then supplied to the vacuum processing chamber 10 by passing theprocess gas through a gap between the window 20 and the gas distributionplate 22. Suitable gas distribution plate arrangements (i.e.,showerhead) arrangements are disclosed in commonly owned U.S. patentapplication Ser. Nos. 08/509,080; 08/658,258; and 08/658,259, thedisclosures of which are hereby incorporated by reference.

The gas distribution plate can have various designs one example of whichis shown in FIG. 2. The gas distribution plate 40 shown in FIG. 2includes eighty-nine holes 41 and four embossments 42 near the centerthereof for providing a passage for supply of process gas between thegas distribution plate and the dielectric window. The gas distributionplate, liner and/or focus ring can be mounted in chambers of the typesdisclosed in commonly owned U.S. patent application Ser. Nos. 08/658,261and 08/658,262, the disclosures of which are hereby incorporated byreference. A complete description of a process for producingsemiconductor substrates for use with the present invention is disclosedin commonly owned U.S. patent application Ser. Nos. 08/722,371, thedisclosure of which is hereby incorporated by reference.

Turning to FIG. 3, an illustration of a TCP™ coil 50 is shown positionedabove a plasma 42 generated by the coil. The outer end of the coil 50 isconnected to a variable terminating capacitor 60 having output voltageand current V_(o) and I_(o), respectively. The inner end of the coil isconnected to an RF power supply which supplies RF power through matchingcircuit 65 such that the inner end of the coil has input voltage andcurrent V_(i) and I_(i), respectively. Matching circuits, such as 65,are known to those skilled in the art and therefore are not furtherdescribed herein. As a lossy transmission line, the TCP™ coil 50exhibits a voltage standing wave that can be displaced along its lengthby varying the terminating impedance of the coil. In the example shownin FIG. 3 the terminating impedance is provided by capacitor 60.However, it will be apparent to persons skilled in the art thatdisplacement of the voltage standing wave can be achieved by othertechniques.

In a region of the coil 50 that exhibits a high voltage standing waveamplitude (i.e., peak voltage) there is a corresponding region of highion bombardment of the gas distribution plate 40. In the region of highion bombardment, the gas distribution plate remains relatively free frompolymer build-up. In other regions where ion bombardment is low,build-up of polymer can occur. In other words, in the regions of thecoil 50 where peak voltage is high, polymer build-up is minimized inadjacent areas on the interior surface of the gas distribution plate.

The location of the highest voltage amplitude of the standing wave canbe displaced along the coil 50 by manipulating the terminatingimpedance. In an exemplary embodiment, capacitor 60 is used to vary theterminating impedance. By varying the value of the capacitance ofcapacitor 60 the regions of high and low ion bombardment can be movedover the surface of the gas distribution plate 40 to effectuate cleaningof the plate 40.

According to an exemplary embodiment of the invention it was found thatwhen the termination capacitor was set at a low value the dielectricmember 70 includes an inner area 72 of polymer build-up and an outerrelatively clean area 74, as shown in FIG. 4a. By raising thecapacitance of the capacitor 60, the area of polymer build-up can beshifted to an outer area 76 and the previous build-up on the inner areacan be reduced by ion bombardment of that area to form a relativelyclean inner area 78, as shown in FIG. 4b. According to the invention,the capacitance can be controlled and/or varied to move the clean zonesover the inner surface of a dielectric member such as the gasdistribution plate in order to inhibit deposit build-up.

The capacitance of variable capacitor 60 can be controlledelectromechanically by a control circuit 68, for example. According tothis embodiment of the invention, at least two control schemes can beimplemented. A first control scheme systematically varies thecapacitance of capacitor 60 during processing such as an etching step oroxygen clean step. According to a preferred embodiment the capacitanceis changed in the same way for each cycle in order to provide uniformand reproducible processing of sequentially processed substrates. Asecond control method operates the capacitor 60 at one value during aprocessing cycle and at a second value during the oxygen clean cycle.Alternatively, in the case where the substrate is smaller in size thanthe antenna, the capacitance could be set at a value which achievesuniform build-up of deposits over an area of the dielectric membercommensurate with that of the substrate. In this case, uniform substrateprocessing can be achieved followed by an optional cleaning step toremove the deposit build-up.

FIGS. 5-8 illustrate details of a circuit which can be used to drive theTCP™ coil shown in FIG. 1. As shown in FIG. 5, the antenna is in theform of a TCP™ coil supplied with RF power through a circuit arrangementincluding capacitors C₁, C₂ and C₃ with a termination capacitor C₄connected to an output of the coil. FIG. 6 illustrates the effect ofvariations of the C₄ value (normalized) of the circuit shown in FIG. 5on normalized values of C₁, C₂ and C₃ which minimize reflected power.For autotuning, C₂ and C₃ can be servo driven and C₃ can be manuallyadjusted. FIG. 7 illustrates the effect of variations of the C₄ value(normalized) on the antenna peak voltage (normalized) and the bottomelectrode RF voltage (normalized). FIG. 8 illustrates the effect ofvariations of the C₄ value (normalized) on the antenna peak current(normalized) and the bottom electrode RF current (normalized). Theoptimized values of C₁, C₂, C₃ and C₄ will depend on many factors suchas the type of substrate to be processed, the type of antenna, the typeof process to be carried out, etc.

The bottom electrode is used to apply an RF bias to the substrate beingprocessed. For example, it is conventional to apply at 4MHz RF currentto the bottom electrode. The RF voltage and current on the bottomelectrode are determined by the reactor impedance with a plasmaestablished in the reactor. FIG. 7 shows that the RF voltage on thebottom electrode is a function of the C₄ value. The changes in RFvoltage on the bottom electrode does not have a major effect onprocessing of a substrate. As shown in FIG. 8, the dc voltage (i.e., RFbias) on the bottom electrode becomes more negative as the plasmageneration is shifted towards chamber walls for higher C₄ values.

It is noted that the embodiments described above have been provided forillustrative purposes only and that other embodiments will suggestthemselves to those skilled in the art. For example, antennas other thanthe flat spiral coil 50 described above may be used to control the ionbombardment of the gas distribution plate. For instance, more than oneantenna can be used and/or the antenna can have non-planarconfigurations or have a non-spiral shape. If a spiral coil is used, thenumber of turns thereof can be selected based on the size of thesubstrate to be processed. In addition, while the invention has beendescribed with respect to controlling the capacitance of capacitor 60,other techniques could be used to control the ion bombardment on the gasdistribution plate. For example, inductors could be used to terminatecoil 50 and provide a variable impedance in accordance with the presentinvention. Also, in addition to or in place of varying the terminationimpedance, the standing wave could be modified by electrical componentsconnected to other portions of the coil (e.g., optional circuitcomponent 90 as shown in dotted lines in FIG. 3).

The foregoing has described the principles, preferred embodiments andmodes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. Thus, the above-described embodiments should be regarded asillustrative rather than restrictive, and it should be appreciated thatvariations may be made in those embodiments by workers skilled in theart without departing from the scope of the present invention as definedby the following claims.

What is claimed is:
 1. A plasma processing chamber comprising:asubstrate holder for supporting a substrate within an interior of theplasma processing chamber; a dielectric member having an exposed surfacefacing a reaction zone adjacent a substrate supported on the substrateholder; a gas supply supplying process gas into the reaction zone; anantenna supplying radiofrequency energy into the interior of the plasmaprocessing chamber and energizing the process gas into a plasma statefor processing a substrate, a portion of the radiofrequency energy beingcapacitively coupled to the plasma by a voltage standing wave; and anelectrical circuit which shifts a location of a maximum voltageamplitude of the voltage standing wave along the antenna such that anamount of ion bombardment on the interior surface of the dielectricmember is varied during sequential processing of substrates in theplasma processing chamber can be obtained.
 2. The plasma processingchamber according to claim 1, wherein the dielectric member comprises agas distribution member and the circuit comprises a terminationcapacitor electrically connected to an output of the antenna.
 3. Theplasma processing chamber according to claim 1, wherein the dielectricmember comprises a dielectric window and the antenna comprises asubstantially planar spiral coil adjacent the window, the coil supplyingthe radiofrequency energy through the window to energize the processgas.
 4. The plasma processing chamber according to claim 1, wherein thedielectric member comprises a gas distribution member, the plasmaprocessing chamber including a dielectric window adjacent an outersurface of the gas distribution member, the gas distribution memberhaving a plurality of gas outlets extending through the exposed surfaceand a plurality of gas distributing channels in the outer surface, theouter surface being in contact with the dielectric window and the gasdistributing channels supplying the process gas to the gas outlets. 5.The plasma processing chamber according to claim 4, wherein thedielectric window and/or the gas distribution member has a substantiallyuniform thickness and substantially planar configuration.
 6. The plasmaprocessing chamber according to claim 1, wherein the circuit includes atermination capacitor connected to the antenna such that less ionbombardment occurs on an inner region of the interior surface of thedielectric member than on an outer region surrounding the inner region.7. The plasma processing chamber according to claim 1, wherein thecircuit includes a termination capacitor connected to the antenna suchthat an amount of ion bombardment changes during processing of asemiconductor substrate, the amount of ion bombardment being greater onan outer region of the interior surface of the dielectric member than onan inner region during an oxide etch part of the processing and theamount of ion bombardment being greater on the inner region than on theouter region during an oxygen clean part of the processing.
 8. Theplasma processing chamber according to claim 1, wherein the circuitcomprises a control circuit and a termination capacitor electricallyconnected to an output end of the antenna, the control circuit varyingthe capacitance of the termination capacitor such that a location of amaximum voltage amplitude of the voltage standing wave along the antennais shifted during processing of an individual semiconductor substrate.